Mitigation of Capacitor-Voltage Fluctuation in a Modular Multilevel Triple-Star Bridge-Cell (TSBC) Converter for Medium-Voltage Motor Drives

This paper provides a theoretical and experimental discussion on a modular multilevel triple-star bridge-cell (TSBC) converter with focus on mitigating the capacitor-voltage fluctuation contained in each bridge cell. This paper assumes that the TSBC converter drives an interior permanent magnet synchronous motor rated at 75Hz loaded at the rated torque. A new control method proposed in this paper is characterized by properly superimposing both common-mode voltage and circulating currents in the whole speed range from 0 to 75Hz. Moreover, this paper optimizes the amplitude and frequency of the superimposed common-mode voltage with the help of theoretical analysis, thus resulting in minimizing the voltage fluctuation. This optimized control method can reduce both the voltage fluctuation and the cluster-current amplitude to values below those of existing methods. Experimental waveforms obtained from a three-phase downscaled model verify good start-up performance from a standstill to the rated speed.